The use of earth station parabolic microwave antennas to receive television transmissions directly from satellites in geosynchronous orbit is now commonplace. At present there are many satellites in geosynchronous orbit transmitting signals, and it is often desirable to direct an earth station antenna at different ones of these satellites in order to receive different transmitted material. Antennas may be provided with a positioning device to enable a user to position the antenna as desired. While simple on/off and directional control of the positioning device permits a user to direct the antenna at different satellites while watching a signal strength indicator to maximize received signal strength, it is often difficult and time-consuming to locate a particular desired satellite in the sky. An automatic positioning system for directing the antenna to a selected one of several predetermined positions in the sky, each of which corresponds to the position of a satellite in geosynchronous orbit, would be desirable.
There are several "programmable" automatic satellite positioning systems presently on the market. Most of these available systems use information stored in a volatile random access memory (RAM) during installation (or programmed into the RAM at time of manufacture) to identify antenna position. The actual position of the antenna is then typically monitored by a pulse transducer (e.g. mounted on an actuating motor shaft) whose accumulated output is compared to a stored number of expected pulses in a register or other storage memory. The antenna is repositioned as necessary to match its actual position with a selected desired position as stored in RAM.
One disadvantage of such presently available automatic satellite positioning systems is that the programmed information (or the actual antenna position information) stored in volatile RAM may be accidently lost. Power must be applied to the system at all times in an attempt to prevent such losses. Nevertheless, unavoidable power failures will cause all information stored in RAM to be lost from time-to-time unless some kind of a "backup" battery power source is provided. Line "glitches" (caused by, for instance, power line noise generated during electrical storms or back EMF created by other devices which happen to be fed by the same power line transformer) can also cause information stored in a RAM memory to be lost or changed. Such occurrences can result in unsatisfactory operation at best, requiring reloading of all of the information stored therein. Other disadvantages include the high cost of components and manufacturing, complexity in design, and problems with reliability. A simpler automatic positioning system making use of commonly-available, inexpensive material and which can store non-volatile user programming of the positions of several satellites would be desirable.
One method of sensing the actual position of a movable antenna with respect to a fixed mount is to employ optical sensors. U.S. Pat. No. 4,077,037 to Bryden (issued Feb. 28, 1978) discloses an antenna resolver for indicating the position of a radar antenna. A cylindrical-shaped resolver housing defining a number of longitudinal slots cut through its periphery at predetermined intervals is coupled to a rotatable shaft the position of which determines the position of a directional radar antenna. Light emitting diodes mounted within the resolver housing continuously emit light outward. Phototransistors are mounted outside of the resolver housing, and generate signals when exposed to light produced by the light emitting diodes which shine through the slots in the housing. A register counts the number of on-off alternations in the signals produced by the phototransistors as the resolver housing rotates in order to determine the position of the radar antenna.
Of course, optical monitors for indicating the position of a rotating shaft are relatively well-known. For example, U.S. Pat. No. 3,746,842 to Fowler (issued July 17, 1973) discloses a digital magnetic compass wherein the position of the compass is monitored by optical means. A Grey-coded disk having plural transparent tracks is mounted on the compass shaft. A light source is positioned on one side of the disk, and plural photocells, one for each track of the disk, are mounted on the other side of the disk. The parallel output from the photocells constitutes a binary coded signal which uniquely identifies the rotational position of the disk with respect to a fixed enclosure. The position of the compass shaft may thus be determined by monitoring the output of the photodetectors. U.S. Pat. No. 4,190,962 to Lyman, Jr. (issued Mar. 4, 1980) discloses an improvement on a digital magnetic compass using an encoding cone in place of a disk. The encoded cone is "trimmed" to eliminate non-linearity of the sensor response by darkening sections of it with black lacquer while lightening other sections by fine scratching of the pattern. An optical shaft angle transducer utilizing a rotating control disk having a plurality of radial segments each having relatively opaque and transparent portions with the proportion of the portions changing from segment to segment to provide an output indicating angular position is disclosed in U.S. Pat. No. 4,109,389 to Balcom et al. (issued Aug. 29, 1978).
The position of a satellite receiving antenna may not, however, be adequately described or determined by the position of a single rotatable shaft. U.S. Pat. No. 3,945,015 to Gueguen (issued Mar. 16, 1976) discloses a satellite tracking antenna movably supported on a steering mounting. The antenna may be controlled to rotate independently about an axis of elevation (up and down positioning) and an axis of azimuth (left to right rotation). U.S. Pat. No. 3,158,861 to Iribe (issued Nov. 24, 1964) discloses an apparatus for monitoring the actual position of a radar dish antenna by focusing a spot of light produced by a light source mounted on the radar antenna onto a planar photoelectric sensor. By determining the position of the spot of light on the photoelectric sensor, the elevation and azimuth of the radar antenna may be determined. U.S. Pat. No. 4,003,055 to Eriksson et al. (issued Jan. 11, 1977) discloses improvements to this apparatus.
U.S. Pat. No. 4,215,410 to Weslow et al (issued July 29, 1980) discloses a solar tracker for controlling motors which drive a solar energy utilizing device about its azimuth and altitude axes to track the sun. The controller has a central processor for inputting data corresponding to, among other things, time of day and the latitude and longitude of the device installation. Memories store program data and tables of data corresponding with the declination of the sun. The processor uses the data to calculate the azimuth and altitude angles of the sun and causes signals to be produced which result in motor controllers causing the motors to turn the solar energy utilizing device through azimuth and altitude axes angles corresponding to the calculated angles.
U.S. Pat. No. 4,333,044 to Blitchington (issued June 1, 1982) discloses a method and system for aligning a device with a reference target. A planar art master contains data marks appearing as darkened areas on an otherwise clear plastic film. To locate the art work with respect to the substrate of a printed circuit board, two servomotors accurately position the artwork with respect to the substrate. An image sensor including lightsensing charge-accumulating elements is used to sense the position of the artwork. U.S. Pat. No. 3,593,030 to Jaskowsky (issued July 13, 1971) and U.S. Pat. No. 3,163,758 to Treacy (issued Dec. 29, 1964) both disclose optical sensors for sensing data on a planar card as it moves rectilinearly.
U.S. Pat. No. 4,154,000 to Kramer (issued May 15, 1979) discloses a remote level sensing instrument employing plural light sources and plural light detectors for indicating the horizontal position of an object by optically sensing the position of a bubble in a spirit level.